Optical metrology systems based on scatterometry techniques are often utilized to characterize critical device layers in semiconductors. Examples of optical metrology systems include, but are not limited to, one dimensional beam profile reflectometry (1D-BPR), two dimensional beam profile reflectometry (2D-BPR), spectroscopic reflectometry, and spectroscopic ellipsometry systems. The foregoing optical metrology systems and others known to the art illuminate samples to perform measurements. Various illumination systems are currently employed to provide illumination to a measurement head of an optical metrology system.
Several currently employed illumination systems suffer from structural or performance deficiencies. Some current systems are not compact and require a large number of optical surfaces to guide or manage illumination delivered to the measurement head. Some current systems are limited to one or more specific laser wavelengths or are unable to provide illumination in certain wavelength ranges to the measurement head. Some current systems are very susceptible to noise, such that even small amounts of noise in voltage driving an electro-optical modulator, such as a Pockets cell, can result in unacceptable amounts of noise affecting intensity of illumination delivered to the measurement head. Some current systems lack stability in coupling from free space to an optical fiber, thus resulting in large amounts of noise affecting intensity of illumination delivered to the measurement head. Some current systems are susceptible to laser speckle from high degrees of spatial coherence produced by a laser point source and/or single-mode optical fiber. The foregoing examples illustrate deficiencies in some of the illumination systems currently known to the art.